1. Field of the Invention
This invention relates to heat resistant aluminum alloys excellent in tensile strength, ductility and fatigue strength, especially notch fatigue strength which alloys are especially suited to use in structural members of internal combustion engines, such as connecting rods and movable valve members (e.g. valve lifter, valve spring retainer, rocker arm, etc.).
2. Description of the Prior Art
As energy-saving measures in automobiles, motor cycles etc., it is strongly requested to lighten the weight of them. Particularly, if structural members of the internal combustion engines, particularly connecting rods become light, significantly improved high-performance engines will be expected. Under such circumstances, it is highly desired to prepare the connecting rods and other parts using aluminum materials.
The connecting rods are ordinarily employed in the temperature range of from room temperature to 150.degree. C., and particularly, in internal combustion engines under a high load, they are employed at nearly 200.degree. C. Therefore, it is required that the connecting rod materials have sufficient tensile strength, ductility and fatigue strength for use in the temperature range of from room temperature to 200.degree. C. Besides such properties, it is also significant that the modulus of elasticity is high and the coefficient of thermal expansion is low. Among such requirements, ductility and notch fatigue srength are especially important.
Even the alloys designated AA 2218 and AA 2618, which are considered to be superior in high temperature strength, are still insufficient in tensile strength and fatigue strength, especially notch fatigue strength (fatigue strength in notched materials) at elevated temperatures of 150 .degree. C. or higher. For this, aluminum alloys have been scarcely used in the connecting rods, etc., and only steel materials have been employed.
However, as described above, since the internal combustion engines are considerably improved in the efficiency by lightening the weight of the structural members, mainly connecting rods, it is still strongly desired to produce the connecting rods or other members from aluminum alloy.
In response to such demands, Applicant's Assignee has previously proposed Al-Fe-V-Mo-Zr alloy materials containing dispersoids whose size is controlled, the alloys being superior in tensile strength and fatigue strength at elevated temperatures (Japanese Patent Application Laid-Open No. 62-238 346).
The above-mentioned materials have been prepared by powder metallurgy techniques. The materials are usually used in the as-forged state or after cutting off flash formed at mating faces of a metal mold by a chipping process. However, in such a surface state, the surface roughness and microcracks constitute notches, and thereby may cause a reduction in fatigue strength. Further, if the connecting rods are subjected to an unusual load, ruptures or breakages will rapidly occur due to lack of ductility. Therefore, the reliability of the parts becomes low.
The aluminum alloy material previously proposed in Japanese Patent Application Laid-Open No. 62-238 346 has a high tensile strength but has a low fatigue strength.